Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

"Ubadah bin Samit and Mu'awiyah happened to meet, either in a church or in a synagogue. 'Ubadah bin Samit narrated to them and said: 'The Messenger of Allah (saw) forbade us from selling silver for silver, gold for gold, wheat for wheat, barley for barley, and dates for dates.'I one of them said: "And salt for salt," but the other did not say it. "And he commanded us to sell wheat for barley, or barley for wheat, hand-to-hand, however we wished."